Mineralogical controls on the bacterial oxidation of refractory Barberton gold ores

The effect of mineralogical characteristics of gold ore minerals on the nature of sulphide oxidation during a bacterial leaching process was investigated. Three different ore types from the South African goldmines were used, i.e. an arsenopyritic‐pyritic ore (Sheba goldmine), a pyritic ore (Agnes goldmine) and a loellingitic‐arsenopyritic ore (New Consort goldmine). Detailed mineralogical characterization of each ore was performed. Thereafter, polished sections of the sulphides were suspended in a bacterial leach pulp in an air‐stirred vessel for various periods of time. The effect of bacterial oxidation on the sulphides was monitored. Different types of gold‐bearing arsenopyrite exist, each type having its own characteristic behaviour during the bacterial oxidation process. The rate of oxidation is controlled by the amount of defects in the crystal structure, and the amount of defects is again controlled by the composition of the arsenopyrite crystal. The distribution of refractory gold in the sulphide minerals can be correlated with the presence of compositional zones and structural deviations. These same mineralogical features also control the sites and rates of bacterial oxidation. Thus. refractory gold occurs at sites which are preferentially leached by the bacteria. The rate of gold liberation from sulphides is therefore being enhanced during the early stages of bacterial oxidation. Defects in a crystal structure influence the rate of bio‐oxidation, and can be related directly to the crystal structure of the sulphide mineral, the crystallographic orientation of the exposed surfaces, and differences in chemical compositional and mechanical deviations in the crytals. A combination of all of these mineralogical factors influences the bacterial oxidation process. To optimize and to understand the leaching of an individual ore it is important to establish its controlling factors.